1. Field of the Invention
This invention relates to ammeters and particularly to a femtoammeter for measuring currents in the range of 10.sup.-15 amperes.
Femtoammeters measure incredibly small currents. Unfortunately, the small currents measured are oftentimes smaller than the noise transients which can be anticipated within the ammeter. Conventional ammeter circuitry can easily give erroneous readings.
Femtoammeters find application in sensitive instrumentations in nuclear power plants such as main steamline radiation monitors, out of core wide range radiation monitors, out of core intermediate range monitors, and area and process radiation monitor functions.
Where ammeters are placed in these locations, they simply cannot be off line for inordinate periods of time. Ammeter reliability must be high; the instruments must not be inoperative due to transients. Moreover, the instrument must not cause false readings, as false readings cause nuclear power plants to go off line and lose operating revenues.
2. Summary Of The Prior Art
Referring to FIG. 1, a typical ammeter circuit is disclosed. An input 14 connects to a leg 15 of an amplifier 16. A constant current source 18 passes through a temperature compensating log diode 19 connected to the negative input of amplifier 16. A log diode 19 connected in the feedback loop 20 of the amplifier 16 controls amplifier output through a digital-to-analog converter 25.
In operation, log diode 19 tailors the output of ammeter 16 so that currents from about 10.sup.-13 to 10.sup.-3 amps may be log plotted against the voltage typically from 0 to 1 volts.
Such circuitry has had difficulty in the femtoammeter environment. First, because of the extremely small currents, normally measured voltage transients reverse bias amplifier 16. When amplifier 16 is reversed biased, only leakage through log diode 19 can reinstate positive bias to amplifier 16. Time constants of reinstatement in the order of 10,000 seconds (two hours, forty minutes) can be anticipated. In many modern instrument environments such recovery periods are wholly unacceptable.